1. Field of the Invention
This invention relates to microstrip antenna structures and more particularly to microstrip antenna arrays which radiate and receive circularly polarized electromagnetic radiation.
2. Background of the Invention
In the past various antenna arrangements have been developed to transmit and receive circularly polarized microwave radiation. A classical arrangement is the horn antenna which is disclosed in European Pat. No. 0,071,069 issued to Werner Lange on Feb. 9, 1983. Lange's microwave antenna includes a horn shaped waveguide and two excitation radiators arranged orthoganally to one another and perpendicular to the axis of the horn waveguide. The excitation radiators are driven from a 90 degree 3 dB hybrid coupler. This antenna arrangement, however, is expensive and difficult to manufacture. Additionally, it is rather large and therefore cannot be used in applications requiring compact transceivers.
Another conventional antenna arrangement is disclosed in U.S. Pat. Nos. 4,180,817 and 4,217,549, issued to Gary Sanford and Bengt Henoch, respectively. Sanford and Henoch disclose a two-dimensional antenna array having a plurality of square radiating elements arranged in rows and columns. Each square radiating element is excited by two signals 90 degrees out of phase which are applied to adjacent sides of the element. Each square radiating element therefore radiates two signals, one of a first polarization and the other of a second polarization. However, since two signals are applied to each radiating element, these two signals tend to cross-couple which may distort the transmitted signals. Additionally, the radiating elements must be exactly square to radiate circularly polarized radiation and not elliptically polarized radiation. This factor can adversely increase manufacturing costs.
In a further development which is in pending application No. 984,526, now U.S. Pat. No. 4,742,354 and assigned to the same assignee herein, a transceiver is disclosed having two linearly polarized antennas arranged orthogonally side by side. However, in certain applications, such as automobile anticollision radar transceivers, it is desirable to have even a more compact antenna arrangement.